FIG. 1 is a circuit block diagram of a conventional motor driving system. As shown in FIG. 1, the conventional motor driving system 1 is electrically connected with an alternating current (AC) power source 2 and a motor 3. The motor driving system 1 may receive an input voltage from the AC power source 2 and convert the input voltage into a driving signal for controlling operations of the motor 3. The driving signal contains an output voltage, an output current and an output frequency. The motor driving system 1 comprises a motor driver 4 and a braking device 5. The motor driver 4 may receive an input voltage from the AC power source 2 and convert the input voltage into the driving signal for controlling operations of the motor 3. The motor driver 4 employs a pulse width modulation (PWM) technology to change the frequency and amplitude of the output voltage of the driving signal, which is outputted from a converter (not shown) of the motor driver 4. Consequently, the rotating speed of the motor 3 is correspondingly adjusted. When the frequency of the output voltage is increased, the motor 3 is accelerated. Meanwhile, the electrical energy is transferred from the AC power source 2 to the motor 3 through the motor driver 4 so as to provide required kinetic energy for accelerating the motor 3. When the frequency of the output voltage is decreased, the motor 3 is decelerated. Meanwhile, the kinetic energy of the motor 3 is converted into electrical energy by the induction generator action. The electrical energy is fed back to the motor driver 4, or the electrical energy is converted into thermal energy to be dissipated away.
For assisting in energy conversion during the deceleration period, the electrical energy fed back to the motor driver 4 is consumed by the braking device 5 during the deceleration of the motor 3. The braking device 5 may be a braking resistor or a braking energy regenerator. The braking resistor is configured to convert the feedback kinetic energy during the deceleration of the motor 3 into the thermal energy to be dissipated away. The braking energy regenerator is configured to convert the feedback kinetic energy during the deceleration of the motor 3 into the AC electrical energy that flows back to the AC power source 2.
However, the braking resistor will increase the cost and may induce some dangerous risks under some kind of environments. For example, in the environment of a large number of inflammable materials, a fire accident will occur due to the excessive heat generated by the braking resistor. On the other hand, if the braking energy regenerator is employed, the cost will be increased a lot. Besides, in the situation of immediately stopping the motor for an emergency event, if the feedback kinetic energy during the deceleration of the motor 3 is not converted or consumed completely, the motor driver 4 will launch self-protection functions to prevent damage. Under this circumstance, the motor driver 4 is readily suffered from shutdown or damage. However, if the braking device 5 is not included in the motor driving system 1, it will take a long time (e.g. 40 seconds) for stopping the motor 3 from a rated speed.
For solving the above drawbacks, a stage-type control method is used to decrease the output frequency in order to decelerate the motor. This stage-type control method can effectively stop the motor in a short time period without externally connecting any braking device. However, the performance of this method depends on the configurations and specifications of the motor. In other words, the optimum operating frequency is varied with the different structure of the motor or circuit layout of the whole system. Therefore, the decrement of the frequency in the stage-type control method is not easy to be determined. Besides, if the frequency decrement is incorrectly set, the deceleration effect will be reduced a lot.
Therefore, there is a need of providing a motor deceleration method and a motor driving system for stopping the motor in a short time period in order to avoid the above drawbacks.